In a conventional data transmission system, a reference time base common to individual terminal stations is provided such that specified times within the cycle period of the above-mentioned reference time base are allocated to the individual terminal stations in a one-to-one correspondence to provide transmission timings unique to the respective terminal stations, as shown in, e.g., JP 8-130774 A. Since each of the terminal stations performs a transmitting operation within the specified time period based on the unique transmission timing mentioned above at the time of data transmission, time-division transmission control can be performed without depending on control from the base station. In this system, each of the terminal stations performs data transmission based on a transmission timing signal therefor based on a reference timing generated by a transmission timing generator. This allows data transmission to the base station without transmission timings overlapping between one terminal station and another. In other words, data transmission does not depend on control from the base station, so that time-division transmission control is implemented.
However, since the reference time base common to the plurality of terminal stations is provided and the specified times within the cycle period of the reference time base are allocated to the individual terminal stations in a one-to-one correspondence, the unique transmission timings cannot be changed dynamically as required.
In the case of transmitting large-amount of data, each of the terminals stations can transmit the data only within the range of the transmission cycle period allocated thereto and cannot effectively use a communication slot not used by another terminal station.
Furthermore, since the reference time base common to the plurality of terminal stations is provided and the specified times within the cycle period of the reference time base are allocated to the individual terminal stations in a one-to-one correspondence, continuous communication longer than the continuous cycle period of the reference time base cannot be performed.
An object of this invention is therefore to provide a time-division synchronous wireless modem device which the efficiency in using communication slots is high.